Image correlator tube with secondary transmission of collimated rays

ABSTRACT

An image orthicon type device is disclosed for electrooptical integration and correlation systems. A photoemissive cathode and a dynode electrode of an electron conductive dielectric material provide for direct emission of secondary electrons from collimated light input signals to impinge on an insulated target member. The normal photoelectric emission current is multiplied by a factor inherent in direct secondary electron emission by the dynode electrode and becomes a linear function of the accelerating potential between the photocathode and dynode for processing time variable modulating signals. An axial focusing magnetic field is utilized. The dynode electrode arrangement may be constructed as a separate sealed compartment in the overall device.

United States Patent 91 Osepchuk [541 IMAGE CORRELATOR TUBE WITH SECONDARY TRANSMISSION OF COLLIMATED RAYS [75] Inventor: John M. Osepchuk, Concord, Mass.

[73] Assignee: Raytheon Company, Lexington,

Mass.

[22] Filed: Aug. 21, 1970 [21] Appl. No.: 65,874

[52] U.S.Cl. ..3l5/ll,3l5/l2,313/65 T,

[51] Int. Cl. ..H0lj 31/48 [58] Field of Search ..3l5/ll, 12;

[56] References Cited UNITED STATES PATENTS 3,424,937 1/1969 Steiner ..3l5/l0 3,476,197 11/1969 Penix et a1. ..3l5/ll X SIGNAL INPUT NO! [451 Apr. 17, 1973 3,496,290 2/1970 Smith ..3l5/ll X Primary Examiner-Carl D. Quarforth Assistant ExaminerP. A. Nelson At!0rney1-larold A. Murphy, Joseph D. Pannone and Edgar O. Rost [57] ABSTRACT An image orthicon type device is disclosed for electrooptical integration and correlation systems. A photoemissive cathode and a dynode electrode of an electron conductive dielectric material provide for direct emission of secondary electrons from collimated light input signals to impinge on an insulated target member. The normal photoelectric emission 2 Claims, 4 Drawing Figures 24 ELECTRON IMAGE SECTION v..-. V77 1 v I fofoiokaicfzfi GNAL INPUT NO. 2

fit; new

ELECTRON MULTIPLIER AND SCANNING SECTION PATENTEUAPRI'IIQB 3,7285.

SHEET 1 OF 2 DYNODE'FOR TRANSMISSION SECONDARY 22 EMISSION MULTIPLIER AND 2 SCANNING SECTION PATENTED APR 1 (I973 SHEET 2 [1F 2 V (kv) PRIMARY ELECTRON ENERGY D m E T M A L M U US D LY ON CA MA C IMAGE CORRELATOR TUBE WITH SECONDARY TRANSMISSION OF COLLIMATED RAYS BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to electrooptical correlation systems and electron discharge devices of the image orthicontype.

2. Description of the Prior Art In the electronic communications art the employment of electrooptical techniques in the processing of information signals results in a mixture of input signals from which are extracted only those integrated signals which correspond to a chosen time sequence pattern. The output comprises a signal varying in time in accordance with a modulated light pattern which may be coupled to any utilization load. Electrooptical integrator devices, particularly those employing cross correlation techniques, are desirable in the processing of information to obtain higher resolution in target definition and location. For the purposes of this specification the term correlation" shall be interpreted to refer to the multiplication of two functions in the processing of intelligence signals plus the integration and readout by electrical means of the signal waveforms. An area for the utilization of the devices under consideration resides in aerial observation and mapping of geographical areas utilizing high frequency electromagnetic waves.

In a system utilizing electrooptical correlation techniques electrical signals are converted by elastic waves within a solid birefringent material and are electrically multiplied by a second input electrical signal which provides a bidimensional replica of the transmitted signal. The resultant product evolved by the correlation process is a time varying electrical signal which may be periodically read out in a TV raster fashion to be recorded or displayed on a monitor tube. An electrical conversion device of interest is the image orthicon tube having the essential components for electrooptical correlation. A photocathode disposed within an evacuated envelope converts the incident collimated optical image rays into an electrical discharge image by means of primary and secondary electron emission. An electron lens system processes these signals by impingement on an insulating target member for integration and correlation. An electron gun and deflection system scanning the accumulated target charge potential is disposed on the reverse side of this target member and also provides an electron multiplier and readout section to yield a correlated signal output. The main problem in utilizing such devices is the means for inserting the second input electrical signal required for the cross correlation techniques.

Prior art attempts to adapt image orthicon electron discharge devices to perform the correlation and integration functions in electrooptical systems have resulted in operation of the target memberclose to its critical voltage or first crossover operating mode" to reduce the charge distributions produced by .uncorrelated signals. Operation at this critical voltage is nonlinear and unstable and attempts in this area have been relatively unsuccessful. Operation at voltages above the critical voltage results in accumulation of positive charges from uncorrelated signals which drastically reduces the usable dynamic range and requires a periodic discharging cycle. In addition, modulation of the electron beam velocity produces drastic disturbances of the electron optical system focus.

A suggestion for achieving high uniformity and stability at the crossover voltages, as well as a high degree of linearity of the secondary emission characteristics is made in relation to the image orthicon devices by the injection of an input electrical correlation signal on one or more of the electrodes in the electron image section of the device. In US. Letters Pat. No. 3,474,286 issued to Rudolf C. Hergenrother, the photoelectric emission current from the photocathode is modulated by an ad'- jacent mesh electrode to which the second electrical correlation input signal is applied. In this operation mode substantially the same voltages are applied as would be used in the operation of the image orthicon as a television camera tube.

A large part of the observed deviations in performance of prior art tubes is believed to result from spurious and complex phenomena such as spacecharge effects, redistribution, target contamination, and transverse leakage or field effects, as well as problems in the provision of clean dielectric target surfaces. The modulation of photoelectron emission current, then, for electrooptical correlation systems by means of intermediate electrode members or the photocathode has still not solved all the problems relating to the accumulation of the charge on a target member to yield the desired integration and correlation output. A need arises, therefore, for another means for introducing modulation of photoelectric emission in devices of the image orthicon type for use in electrooptical correlator systems. The modulation desired should produce both positive and negative charge effects on the target with a relatively stablecrossover point to minimize distortion of the image.

SUMMARY OF THE INVENTION In accordance with the teachings of the present invention multiple photoelectric emissive surfaces are disclosed on the electron image side of a device substantially similar to the image orthicon tube. The incidence of collimated input light rays results in the emission of primary electrons from a conventional photocathode structure. An intermediately disposed dynode electrode member of an electron conductive dielectric material provides for the direct generation of a-profusion of secondary electrons by transmission processes. The secondary electron emission is a linear function of the primary electron energy and a low density material is preferred to give uniform yields without instabilities found in surfaces emitting secondary electrons by the process of reflection. A multiplication factor is thereby introduced in the photocathode photoelectric emission current by the dynode arrangement within the photoelectric conversion and electron image region of the image correlator tube. The resultant image on a targetmay be scanned in a manner similar .to image orthicon devices for electron multiplication and correlation. An axial magnetic field of sufficient strength is provided for good resolution and focusing of the electrons within the electron image and scanning sections. An electrical correlation signal is impressed uponthe dynode electrode to result in an image signal on a target member having linearity, as

well as uniformity at or near the crossover correlation modes of operation.

Potassium chloride or any halides on an alumina substrate or thin film magnesium oxide, as well as alumina and aluminum chloride may be employed for the dielectric dynode material. Linearities far in excess of those attained in prior art image correlator tubes utilizing reflected secondary electrons have been achieved utilizing the dynode arrangement. Further, secondary electron redistribution problems for electrooptical correlation operations are substantially minimized to reduce the requirements for front target mesh electrode members, as well as rear target mesh members, to provide a slight charge potential on the target electrode member which is scanned by the electron beam on the reverse side of the target member.

Numerous other advantages arise with the disclosed structure in that higher voltage potentials may be employed in the electron image section, as well as stronger than normal magnetic fields to thereby achieve improved focusing and higher resolution of the correlated output signals. Several embodiments of the invention are disclosed including the provision of a separate sealed compartment housing the dynode electrode and target members to prevent contamination by the photocathode material.

BRIEF DESCRIPTION OF THE DRAWINGS The invention, as well as the details for the provision of preferred embodiments, will be readily understood after consideration of the following detailed description and reference to the accompanying drawings, wherein:

FIG. 1 is a detailed diagrammatic illustration of an embodiment of the invention;

FIG. 2 is a diagrammatic view of an electrooptical correlator system embodiment of the invention;

FIG. 3 is a graph illustrating the secondary electron yield characteristics of potassium chloride dynode material as a function of primary electron energy; and

FIG. 4 is a diagrammatic view of an alternative embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, FIG. 1 illustrates the embodiment of the invention as an image correlator tube 22 for electrooptical signal integration and correlation. Before proceeding to the detailed description of the tube it will be of assistance to an understanding of the invention to refer to FIG. 2 showing the electrooptical processing system incorporating the embodiment of the invention. Radiant energy emanating from a source 2 is directed by means of lens 4 to form a collimated beam of rays 6 through an optical delay line modulator 8. The radiant energy beam is focused first through a light polarizer 10 upon a solid state birefringent optical delay line 12, preferably of a material such as fused quartz capable of supporting elastic waves upon stimulation by electrical input signals applied to terminal 14. The delay line element 12 comprises an input transducer 16 of the piezoelectric type coupled to terminal 14 for application of an external time varying pattern of electrical correlation signals from an appropriate source. The opposing end of the optical delay line 12 is terminated in a sonic energy absorber 18 to prevent the generation of undesirable reflections of the traversing acoustic energy.

A second light responsive means follows the optical delay line modulator 8 and comprises an analyzer or cross polarizer 20. The emergent modulated light beam impinges upon a photoemissive cathode of the image correlator tube 22 embodying the teachings of the present invention. The impinging energy results in the emission of primary photoelectrons within the electron image section 24. This section is followed by multiplier and scanning section 26 to collectively provide by electrical converting and translating means the multiplication, integration and correlation functions required for the use intended.

Axial magnetic field focusing means disposed external to envelope 38 of the image correlator tube 22 includes a solenoid coil 28 used to align the scanning beam initially. A deflecting magnetic coil 30, as well as a focusing magnetic coil 32 provide for the focusing of the electrons within the multiplier and scanning section, as well as the electron image section. TI-Ie solenoid means 28, 30 and 31 are biased in a conventional manner utilizing sawtooth voltage generators to provide the focusing and scanning functions with appropriate frequencies and variable voltages.

Referring again to FIG. 1 the electron image section 24 includes a photocathode 34 having a photosensitized surface capable of emitting primary photoelectrons which are focused by a magnetic focusing field producing means 32, as well as an annular anode ring electrode 36 within the envelope 38. The photocathode is conventionally biased at a high negative voltage potential by a suitable voltage supply 40 to control the initial primary photoelectric emission current induced by the impingement of the modulated and collimated light rays 6. The annular anode focusing ring electrode 36 is biased at a somewhat less negative voltage potential by means of supply 42 coupled to. this member.

In accordance with the invention multiple photoelectric emissive converting means are provided within the electron image section 24. A dynode electrode 44 of an electron conductive dielectric material capable of generating. secondary electrons upon impingement of the primary electrons on one side is supported within the electron image section 24 by means of supports 46. The transmitted secondary electrons from dynode electrode 44 are of considerable intensity and impinge upon a dielectric target member 48 which is scanned on the reverse side by an electron beam generated within the scanning section 26.

Numerous examples of an electron conductive dielectric material include magnesium oxide, as well as aluminum and potassium chloride or any halide materials deposited on an alumina substrate. The thickness of the dynode electrode may be empirically determined, however, such a member of approximately 10 to 12 microns in thickness will provide secondary electron emission ratios at levels approximating 3 to 4 times and higher the secondary electron emission ratios by reflected secondaries. In the illustrative embodiment a target mesh member 50 has been shown although in exemplary embodiments of the invention due to the multiplication factor introduced by the dynode electrode member in the photoelectric emission current such mesh members may be found to be unnecessary in achieving the desired crossover operation in electrooptical correlation systems. Where such mesh members are employed a potential of a few volts positive with respect to the dielectric target member 48 surface may be provided by voltage supply 52 relative to the cathode 54 of the scanning gun assembly 56 which is biased along with the internal conductive coating focusing means 58 by voltage supply 60. In view of the intensity of the photoelectric emission current within the electron image section and the illumination of the reverse side of the dielectric target member 48, it may be desirable to darken the scan side of the dielectric target member to absorb any light from the scan beam. This member 48 is biased at near ground potential and the correlation process is derived through the multipli cation and scanning of the target member.

Section 26 comprises a low velocity electron scanning beam 62 which emanates from the gun assembly 56. The impingement of the photoelectric emission current on the image side of the dielectric target member results in a deposition of, desirably, a positive charge of a few volts on the image side. Scanning beam 62 deposits negative electrons in the areas corresponding to the positively charged areas of the electron image. The returning scanning electron beam 64 contacts the anode member of the gun assembly 56 to be scattered, deflected and collected by a plurality of multiplying electrodes 66 together with collecting electrodes 68 for generating the correlated output signals coupled by means of lead 70 to an external utilization monitor. The correlation process from the combination of a mixture of signal inputs and chosen time sequence pattern signals provides a single output correlation signal integrated from the input signal mixture. The important function of the image correlator device is to translate both the positive and negative effects of the combined signals in such a manner as to provide clear resolution of the objects under observation. The stability and uniformity of the signals deposited by the photoelectric emission process on the dielectric target member, therefore is determined by the transmission processes with the electron image section emanating upon impingement of the incident radiant energy rays. THe stabilization or equilibrium of the target member potential provides the basis for the adaptability of the embodiment to electrooptical correlator systems.

The input electrical modulating signal to be correlated denoted by the symbol V(t) is introduced on the dynode electrode member 44 by inductive coupling means 72 from the source 74. The signals in the modulation of the transmitted secondary electrons provide a replica of time varying signals which when combined with the optically delayed collimated signals provide the multiplication and integration process desired. The positive charges deposited on the target member will require a greater number of electrons during the scanning operation for neutralizing such charges. These portions of the target member indicate the initially more intensely illuminated areas of the optical image and require a substantially greater number of electrons than the darker observed areas. As a result, the return scanned beam modulation in the correlated signal output will be the difference between the initial uniform electron beam 62 and the current extracted by this neutralizing process to result in a negative signal relation to he optical image impinging on the photocathode 34.

The instabilities and lack of uniformity in prior art techniques for such image correlator devices is measurably improved by the multiplication factor introduced through the disposition at an intermittent point within the electron image section of the dynode electrode of an electron conductive dielectric material to emit secondary electrons by transmission processes. The normal photoelectric emission current i, is multiplied by the characteristics of the material comprising the dynode electrode 44. The accelerating voltage potentials on the primary electrons in this region of the electron image section will be the photocathode voltage V plus the voltage V(t) impressed on the dynode electrode from the source 74. If we assume that the region between the photocathode and the dynode has a transmission coefficient factor 0 and the photocathode surface has an emission factor k the resultant photoelectric emission current through transmitted secondary electron emission from the dynode electrode impinging on the target member will be i, which may be derived from the following equation:

The multiplied photoelectric emission current i, from the photocathode is collected on the dielectric target member as a negative charge. Reflected secondary electron emission is suppressed by the potential in the region between the dynode electrode and the dielectric target and this inherent suppression renders the utilization of mesh electrodes disposed in front of the target member as an optional item. The impinging correlation pattern signal on the photocathode l(x,y,t) is transmitted through the transparent dynode electrode and as a result a signal of considerable intensity falls upon the dielectric target member. There are, therefore, two sources of charge buildup of opposite polarity on the dielectric target member relying solely on the transmission effect of secondary electrons and not the reflection of such electrons.

The charging rate of the signals on the target member may be derived by the equation:

An indication of the multiplication factor adhieved in the embodiment of the present invention may be noted in FIG. 3. This graph indicates for an electron conductive material such as potassium chloride the secondary electron yield for varying collecting voltage settings relative to the primary electron energy. The ratio obtained, for example, at a collecting voltage Vc of 50 volts as shown by curve 76 can be as high as 16 while the curve at even higher voltages 78 indicates secondary electron emission ratios as high as 40. Curves 80, 82 and 84illustrate intermediate ranges for the potassium chloride material. The high yields from the low density electron conductive materials, therefore, will contribute to provide for uniform yields without instabilities to create redistribution problems. With the aforedescribed dynode electrode arrangement no DC drift or error signals have been observed which previously plagued image correlator tubes utilizing mesh electrode members in the electron image section for the impressing of the input electrical correlation signal. The crossover correlator modes of operation for target electrode members which also raised problems in the utilization of prior art image orthicon type devices in electrooptical correlation systems has been measurably alleviated. In addition, the utilization of higher potentials on the photocathode, as well as dynode electrode, coupled with the use of stronger than normal magnetic fields has resulted in improved focusing and higher resolution in the resultant correlated signals.

FIG. 4 illustrates an alternative embodiment of the invention with structure similar to the described embodiment being similarly numbered. Contamination of,

the dynode electrode surface emitting secondary electrons, as well as the target member 48, may be prevented within electron image section 24 by means of a partition member 88 sealed by conventional techniques to transparent envelope 38. Another possibility for the modification of the invention exists in the disposition of a target mesh member 90 on the opposing side or scan side of the dielectric target member 48. Such an optional mesh member may assist in the application of modulation voltages superimposed on the rear target member surfaces to control the scanning charge distribution during the image writing and reading cycles.

There is thus disclosed an efficient and highly useful image correlator device for electrooptical systems which has displayed both linearity and uniformity in the conversion of the impinging modulated and collimated image rays on a scanned target member. The multiple photoelectric emission means within the electron image section provide a high multiplication factor relative to prior art correlator devices. Numerous modifications, alterations and variations will be readily discernible by those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims. It is intended, therefore, that the preferred embodiments shown and described herein be considered as illustrative only and not in a limiting sense.

What is claimed is:

1. An electrooptical correlation system comprising:

means to collimate and direct radiant energy signals in a beam to traverse an acoustic optical delay line modulator;

said modulator having means for providing a first input electrical correlation signal to modulate the emergent signal beam pattern;

photoelectric conversion and translation means for correlating said beam signals comprising:

an image correlator tube having an electron image section disposed at one end;

a photoemissive cathode for emitting photoelectrons disposed at one end of said section;

a target member disposed at the opposing end of said section;

a dynode electrode member of an electron conductive material disposed between said photocathode and said target member;

means for biasing said dynode electrode member with a second input electrical correlation signal;

and means for electronically scanning the reverse side of said target member, collecting and integrating the product of all said electrical signals to derive a correlated electrical output signal.

2. An electrooptical correlator system according to claim 1 wherein said dynode electrode member is selected of a material from the group consisting of magnesium oxide, aluminum chloride and potassium chloride. 

1. An electrooptical correlation system comprising: means to collimate and direct radiant energy signals in a beam to traverse an acoustic optical delay line modulator; said modulator having means for providing a first input electrical corrElation signal to modulate the emergent signal beam pattern; photoelectric conversion and translation means for correlating said beam signals comprising: an image correlator tube having an electron image section disposed at one end; a photoemissive cathode for emitting photoelectrons disposed at one end of said section; a target member disposed at the opposing end of said section; a dynode electrode member of an electron conductive material disposed between said photocathode and said target member; means for biasing said dynode electrode member with a second input electrical correlation signal; and means for electronically scanning the reverse side of said target member, collecting and integrating the product of all said electrical signals to derive a correlated electrical output signal.
 2. An electrooptical correlator system according to claim 1 wherein said dynode electrode member is selected of a material from the group consisting of magnesium oxide, aluminum chloride and potassium chloride. 